Means and techniques for simulating flight and control of aircraft



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MEANS AND 'mcumquas FOR smuurmc FLIGHT AND cou'moz. 0F AIRCRAFT 14Sheets-Sheet 12 Filed Jan. 11. 1952 Dec. 8, 1959 A. G. VAN ALSTYNE MEANSAND TECHNIQUES FOR smuurmc; FLIGHT AND CONTROL OF AIRCRAFT 14Sheets-Sheet 13 mw w INVENTOR. /7Lw v 60v l/l7/Y/7LSTYNE BY f HTTOENEYSFiled Jan. 11, 1952 United States Patent MEANS AND TECHNIQUES FORSIMULATING FLIGHT AND CONTROL OF AIRCRAFT Alvin Guy Van Alstyne, LosAngeles, Calif., assignor to Gilfillan Bros. Inc., Los Angeles, Calif.,a corporation of California Application January 11, 1952, Serial No.265,978 16 Claims. (Cl. 343-11) The present invention relates to meansand techniques for simulating the flight of aircraft and in particularrelates to the production of signals for application to an intensitycontrol electrode of a cathode ray tube in an automatic ground controlapproach (AGCA) system for producing a visual representation on suchcathode ray tube of the flight of a simulated or artificial aircraftfor, in general, aligning and testing the AGCA equipment; although, itis evident that the arrangement and concepts incorporated herein may beused for other similar purposes.

The arrangement described herein serves to develop signals simulatingthose received from actual aircraft in their flight in the approach zoneto an aircraft landing field, and such simulated signals, for purposesof convenience and reference, are referred to as artificial aircraftsignals.

The present arrangement therefore contemplates means and techniques fordeveloping electronic artificial aircraft signals having thecharacteristics of a real aircraft as seen" by the AGCA equipment, andthe presentation of these signals to produce an artificial aircraft onthe cathode ray tube indicator.

The artificial aircraft thus produced on the cathode ray tube indicatoris caused to respond to control signals developed in the AGCA system ina manner simulating the behavior of a real aircraft. Such conditions ascrosswind, trim tab adjustment, and various response times are availableto simulate typical flight situations.

Many possibilities exist, in use of the apparatus described herein, inthat, for example, the artificial aircraft as viewed on the cathode raytube may be made to stop in mid-air and still seek a predetermined idealglidepath and course line. Further, the artificial aircraft is made tofly at any speed, including reverse, and is set up to accept and becontrolled either by airborne error rate computation or ground ratecomputation.

For purposes of analyzing and appreciating the problems involved in theproduction of the artificial aircraft arrangement herein, it it notedthat in the AGCA system, the actual aircraft being tracked andcontrolled is equipped for control purposes, with an autopilot connectedto an airborne localizer receiver and that when such aircraft is flyingto the left of the runway or ideal course line and is given a correctiveturn signal to the right (assuming the airborne rate computer to beturned off), the aircraft goes into a bank or rate of turn which isproportional approximately to the amount of corrective turn signalapplied to the autopilot; and the aircraft maintains this rate of turnso long as the control signal is applied.

During the time the turn is made, the heading or orientation of theplane is continuously changing so, for purposes of mathematicalanalysis, the change of heading or aircraft orientation is regarded asthe time integral of the turn signal. The new heading imparted by thecontrol signal to the aircraft, however, does not immediately place theaircraft on the proper course with respect to 2,916,736 Patented Dec. 8,1959 ICC which the control signals are computed, for again, it takessome time for the aircraft to traverse the distance from its initialposition to the desired position. The aircrafts displacement from therunway, or ideal course line, is thus considered mathematically as thetime integral of the planes heading with respect to the glidepath.

Thus, to simulate an aircraft which is controlled by turn signalsapplied thereto, two integrations are involved. The first integrationcorresponds to the aircrafts assuming progressively changing headings ororientations during the application of a turn signal; and the secondintegration corresponds to the change of displacement resulting fromheadings or orientations, other than the runway or ideal course lineheading.

The above circumstances are based on the assumption that the correctivesignals transmitted from the ground based radar equipment to theaircraft are so-called error plus error rate signals, or turn signals,for when the aircraft reaches such a heading or orientation as to rendera satisfactory rate of closure to the ideal course, the ground basedcomputing equipment causes the transmitted turn signal to be zero. Theaircraft retains new heading and is eventually turned in the oppositedirection.

On the other hand, when signals are transmitted to the aircraftrepresentative of pure displacement of the aircraft from the idealcourse line, the previously mentioned two integrations are one too manyand an oscillatory condition exists. This oscillatory condition resultssince the control signal is not returned to zero until the course iscrossed, at which time the heading or orientation of the aircraft is toolarge and the plane "over-shoots the ideal course line with reference towhich the control signals are computed.

When pure displacement signals are transmitted, it may be demonstratedthat only one integration, instead of two integrations, is involved,since the output of a true integrator changes at a rate proportional tothe input voltage applied thereto. Thus, if the input voltagecorresponds to displacement, then the output of the integratorapproaches the desired value asymptotically, since the input voltagereaches zero as the error reaches zero.

For simulating these different conditions, the arrangement described andclaimed herein in general, uses a closed control servo loop involvingdouble integration when the signals sent to the aircraft are consideredto be error plus error rate signals, or turn signals; and, when thecontrol signals are pure displacement signals, the arrangement is suchthat a rate network is added to dampen the oscillatory condition whichwould otherwise exist.

This rate network may be considered either airborne or on the ground,but it serves essentially to cancel one stage of integration in theservo loop, namely, the turn to heading integration, for the transientcondition. When one integration is removed from the servo loop, anon-oscillatory system that does not require a rate damping networkresults; indeed, the servo loop becomes oscillatory if a rate dampingnetwork is used.

It is observed that, in the control of a physical aircraft, the firstintegration is not eliminated in an attempt to obtain pure displacementcontrol, a rate device being used for that purpose. In the artificialaircraft arrangement however, airborne rate is simulated, simply andexpeditiously by omitting the first integrator, since omitting anintegral accomplishes a stable condition in much the same way as addinga derivative in the closed servo loop.

It is therefore an object of the present invention to provide means andtechniques whereby the above in dicated results are obtained in a simpleand expeditious manner.

A specific object of the present invention is to provide improved meansand techniques for simulating an actual aircraft in its flight throughthe approach zone of an aircraft landing field and to control thesimulated or artificial aircraft in accordance with control signalswhich would be developed by an actual aircraft assimilated by theartificial aircraft.

Another specific object of the present invention is to provide animproved arrangement for testing and aligning the AGCA apparatus.

Another specific object of the present invention is to provide improvedmeans and techniques whereby a triggering voltage is developed withincreasing or decreasing time delays for application to an intensitycontrol electrode of a cathode ray tube to simulate the progress of theflight of an aircraft.

Another specific object of the present invention is to provide improvedmeans and techniques whereby artificial aircraft is visually reproduced,such artificial aircraft being manually shifted in apparent elevation"and azimuth" angular positions, caused to pursue a rapid return" inrange and cause to fly either backward or forward.

Another specific object of the present invention is to provideartificial aircraft producing means which is connectable as a portion ofa servo alignment loop, in which a channel of automatically controlledflight tracks and controls the artificial aircraft; and, wherein controlsignals developed in the computer unit of the AGCA equipment areintegrated and used to control the elevation and azimuth" positions ofthe artificial aircraft.

Another specific object of the present invention is to provide apparatusof the character mentioned in the preceding paragraph, in which theeffect of such correction control signals may be observed inrelationship to an ideal glidepath and course line, each of which aredisplaced visually with the artificial aircraft controlled by suchcontrol signals.

The features of the present invention which are be lieved to be novelare set forth with particularity in the appended claims. This inventionitself, both as to its organization and manner of operation, togetherwith further objects and advantages thereof, may be best understood byreference to the following description taken in connection with theaccompanying drawings in which:

Figure 1 shows in schematic form apparatus for scanning the approachzone to an aircraft landing field with related circuitry for producing avisual indication of the character illustrated in Figure 6; also, thisapparatus serves to develop information such as azimuth angle voltage,elevation angle voltage, video, blanking voltages and azel relayvoltages used in the automatic ground control approach AGCA systemillustrated in Figure 7.

Figure 2 shows azimuth, beam angle voltage, elevation beam anglevoltage, as well as inverted elevation beam angle voltage and theirvariations with respect to time as developed by the apparatus shown inFigure 1.

Figure 3 shows a cycle of operation of the radar scanning and indicatingarrangements in Figure 1 and serves to illustrate the period duringwhich the azel relay voltage is available.

Figure 4 illustrates other voltages developed during cyclical operationof the apparatus illustrated in Figure 1.

Figure 5 illustrates more details of the cathode beam centering meansshown in block form in Figure 1. Such circuitry being effective to shiftthe displays in Figure 6 sequentially from one origin position 0-1 tothe other origin position 02 and from 0-2 to 0-1, etc.

Figure 6 illustrates the display obtained using the apparatusillustrated in Figure 1, the elevation and azimuth displays beingproduced sequentially on a time sharing basis.

Figure 7 is a block diagram of an AGCA system embodying features of thepresent information which is supplied with certain information developedby the ap paratus illustrated in Figure 1.

Figure 8 illustrates in schematic form circuitry of the video shaperwhich is indicated as such in Figure 7 and which is indicated in blockdiagram form in Figure 14.

Figure 9 illustrates the circuitry of the one-tenth cycle per secondsawtooth generator which is also illustrated as such in block form inFigure 7, such sawtooth generator producing a sawtooth voltage wave ofthe character illustrated in Figure 11, which is used during theso-called search function of the AGCA equipment, it being noted that thecircuitry of Figure 9 is illustrated in block form in Figure 10.

Figure 10 illustrates in block diagram form the circuitry illustrated inFigure 9.

Figure 11 illustrates the sawtooth wave form developed by the apparatusillustrated in Figures 9 and 10.

Figure 12 illustrates in block diagram form the circuitry of the AGCAtracking unit indicated as such in Figure 7, such circuitry beingillustrated in detail in Figure 13.

Figure 13 represents in schematic form the circuitry of the AGCAtracking unit illustrated in Figures 7 and 12.

Figure 14 illustrates in block diagram from the circuitry of the videoshaper, the circuitry of which is illustrated in Figure 8, the videoshaper also being indicated as such in Figure 7.

Figure 15 serves to illustrate the visual indication obtained, of anaircraft being tracked with the bracketing index marks in one instancebeing limited by angle gating, while in the other instance beingextended in the absence of angle gating.

Figures 16 and 17 illustrate in block diagram form different elements ofthe AGCA system and their functional inter-relationship when the systemis adjusted respectively to use ground rate and air rate information.

Figures 18A and 18B interconnected as illustrated, constitute Figure 18,which is a schematic representation of the apparatus in the angletracking and computer unit illustrated as such in Figure 7, suchcircuitry of Figures 18A, 18B, being illustrated also in block diagramform in Figure 19.

Figure 20 illustrates the character of the stretched video or video onsignal, such signal constituting in general an elongated wave having atime duration equal to the time during which radar hits are being madeon an aircraft plus a fixed time interval in the order of S00microseconds.

Figure 21 illustrates the geometrical conditions existing in the azimuthplane, with the radar equipment located adjacent the runway center lineand in relationship to the touchdown point, such figure being useful inappreciating features of the computer illustrated in Figures 18A, 18B,and Figure 19.

Figure 22 is useful in explaining the manner in which the azimuth andelevation beam angle voltages are modified as a function of range forpurposes of comparison with a reference voltage developed in thecomputer unit.

Figure 23 illustrates the manner in which the circuitry in the trackingunit and computer unit is modified so as to provide a visualreproduction on the cathode ray tube of both the azimuth course line andelevation glidepath, which are computed by using thyrite elements in thenormal operation of the computer unit.

Figure 24 illustrates in schematic form the circuitry of a cursorgenerator useful in the production of the glidepath course line andrunway course line illustrated as such in Figure 6, such circuitry beingillustrated in block diagram form in Figure 25.

Figure 25 represents in block diagram form circuitry of the cursorgenerator illustrated in schematic form in Figure 24 and incorporated inthe unit designated AGCA Qurser Generator and Artificial Aircraft" unitin Figure 7.

Figure 26 represents the means for obtaining so-called clutter gating,such circuitry in general being eflective to desensitize the function ofthe computer unit during that period of time while a track aircraft isin a clutter area, the gates produced by the apparatus of Figure 26being illustrated in Figure 27 and the desensitizing effect produced onthe control signals developed in the computer unit being illustrated inFigure 28.

Figure 27 serves to illustrate the positioning of the clutter gatesdeveloped by the apparatus of Figure 26 in relationship to clutterareas.

Figure 28 illustrates in graphical form the manner in which the aircraftcontrol signals developed in the computer unit are modified as a resultof the function of the apparatus illustrated in Figure 26.

Figures 29 and 30 represent respectively in schematic and block diagramforms circuitry of the so-called artificial aircraft unit indicated assuch in Figure 7 in the unit designated, AGCA Cursor Generator andArtificial Aircraft, and is important in illustrating features of thepresent invention.

The artificial aircraft unit illustrated in Figures 29 and 30 embodiesimportant features of the present invention and a description of thearrangement shown in these two figures follows a description of theautomatic ground control approach system.

The apparatus in Figures 29 and 30 is described hereinafter under theheading of Artificial Aircraft illus trated in Figures 29 and 30.

Figures 31-35 illustrate features of the range and angle trackingcircuits and are described herein under a separate heading.

Means shown in Figures 1-5 for producing information useful in producingboth visual indications and tracking The apparatus shown in Figure l isconnected both to the apparatus shown therein for producing visualindications on the face of a cathode ray tube 11 of the character shownin Figure 6 and for also supplying certain data to the automatictracking apparatus shown in block diagram in Figure 7. In Figure l, thesynchronizer 31 serves to generate timing pulses which are used to timethe application of pulses to the transmitter 33 to initiate itsoperation. The transmitter stage 33, pulsed at a constant repetitionrate of, for example, 2000 or 5500 pulses per second consists of, forexample, a magnetron oscillator with a characteristic frequency of about10,000 megacycles. The output of the transmitter stage 33 is transferredto either the elevation (e1) antenna 103 or the azimuth (az) antenna'55, depending upon the position of the motor driven interrupter orradio frequency switch 101. The transmitreceive (TR) switch 97 preventspower from the transmitter 33 from being applied directly to thereceiver 57. This transmit-receive switch 97, as is well known in theart, allows low intensity signals, such as a train of resulting echosignals received on the antennas 103, 55, to be transferred to the inputterminals of the receiver 57. This deflection of energy from thetransmitter 33 to the antennas S5, 103, accomplished by operation ofswitch 101, occurs at a rate of approximately two per second so that ineffect the component antennas obtain four "looks" per second of thespace scanned.

The resulting antenna beams are caused to move angularly, i.e., to scanupon rotation of the shaft 93. The switch 101 is rotated twice persecond, and while energy is being transmitted to one of the antennas 55,103, the resulting electromagnetic beam projected into space is causedto scan such space. The means whereby such scanning movement of theprojected electromagnetic beam is obtained may be of the type describedin the copending application of Karl A. Allebach, Serial No. 49,910.filed September 18, 1948, now Patent Number 2,596,113, granted May 13,1952; for bridge type precision antenna structure, which depends for itsoperation on the use of a variable wave guide type of antenna. Thisparticular means, per se, forms no part of the present invention, and sofar as the aspects of the present invention are concerned, the antennascanning beam may be produced by moving the entire antenna through arelatively small arc of a circle. Actually, in fact, the azimuth antennabeam may scan first in one direction and then in the other, waitingafter each scan while the elevation beam completes a scan in elevation.The azimuth antenna 55 scans a fixed horizontal angle of 20, and is soplaced as to include the approach course to a given airfield runway.Vertical scan of the elevation antenna 103 is from minus one degree toplus 6 degrees.

While in any position during the part of the cycle in which the relayfrequency switch 101 allows the flow of energy into the elevationantenna 103, the elevation antenna beam is electrically scanned inelevation. The position of the elevation antenna beam is measured bymeans of a variable capacitor 59, one plate of which is attached to thebeam scanner of elevation antenna 103 and varied in accordancetherewith, such capacitor 59 comprising one part of a capacitivepotentiometer and contained in the angle coupling unit 85 which may beof the type described and claimed in the ccpendlng patent application ofGeorge B. Crane, Serial No. 212,114, filed February 21, 1951, now PatentNo. 2,650,358, granted August 25, 1953. The angle coupling unit 85 thusused with capacitor 59 is useful in developing the elevation beamvoltage represented as 61 in Figure 2.

Similarly, the angle in azimuth of the radiated azimuth antenna beam ismeasured by the angle capacitor 65 in the azimuth angle coupling unit63A, operating synchronously with the scanner of the azimuth antenna 55.Such variation in azimuth angle voltage as a function of the particularangular position of the azimuth antenna beam is represenetd bycyclically varying voltage 63 shown in Figure 2. It is observed thatthese voltage variations Nos. 61 and 63 have portions thereof shown inheavy lines, and it is these portions which are used to effect controloperations and which are selected by means mentioned later. Figure 2also shows inverted azimuth elevation beam angle voltage as representedby the oblique lines 67A.

Also coupled to the scanner of the elevation antenna 103 is theelevation unblanking switch 67, which has one of its terminals connectedto the continuous voltage source 91 for purposes of developing anelevation unblankiug voltage gate, shown in Figure 4, so timed that itspositive value corresponds to the time of effective scanning of theelevation antenna beam. The azimuth unblankiug switch 65A is similarlycoupled to the scanner of azimuth antenna 55 with one of its terminalsconnected to the continuous voltage source 65B for purposes ofdeveloping azimuth unblanking voltage (Fig. 4) so timed that thepositive portions of such voltage corresponds to the time of effectivescanning of the azimuth antenna beam. Relay switch 69 operates atsubstantially the same time as switch 65A, and synchronously therewithand serves to generate the socalled az-el relay voltage or gate (Fig.4), which is so timed that its positive portion begins at a time justprior to the beginning of the azimuth unblanking voltage and just afterthe end of elevation unblanking voltage, and which ends at a time justafter the ending of the azimuth unblankiug voltage and just prior to thebeginning of the elevation unblanking voltage, all as seen in Figure 4.

Figure 3 shows a schematic diagram of the time relations involved in ascanning action which, typically, occupies a time in the order of onesecond. Forward progress of time is represented by clockwise motionabout this diagram. The central circular region of Figure 3 marked Nshows the time schedule of the scanning operations of the two systems,opposite quadratures being scanned by the same system but carried out inopposite directions. The shaded areas (each comprising approximatelydegrees of the complete 360 degree cycle) represent the periods duringwhich the transmitter 33 is switched by the switch 101 in Figure 1 fromone antenna to the other antenna. Unshaded areas of region N representthe time periods during which one or the other of the antennas is inuse, sending out radio frequency pulses and received reflected echosignals from objects within the field of coverage of the beam. Shadedareas indicate inactive periods during which switching takes place, bothantennas being momentarily isolated from the transmitter and receiver.

The inner annular region M of Figure 3 represents the time schedule ofthe related azimuth and elevation displays, subject however to patternclipping described later, and corresponds to the cyclical variations ofazimuth and elevation voltages represented in Figure 2.

The outer annular region of Figure 3, marked L, shows the time scheduleof currents through the various coils of a number of so-called az-elswitching relays for eflecting time sharing. The relay actuating currentis obtained by the switch 69 (Fig. 2) operating in synchronism with themechanism producing azimuth antenna beam scanning.

More specifically, in Figure l, the wave guide transmission line 79leads from the transmitter 33 and receiving system 97, 57. A T-joint 71divides this transmission line into two branches 73 and 95, leadingthrough switch assembly 101 to the elevation and azimuth assemblies 103and 55, respectively. These branches have suitably placed shutter slotswhich receive the rotating shutters 75 and 75A, respectively. These aremounted on the common drive shaft 93, driven by the motor 77, and havetwo blades each arranged in opposite fashion, so that when one antennatransmission branch is opened, the other will be blocked by its shutter.The shutter blades cover angles of approximately 100 degrees, leavingopenings of 80 degress as required by region N of Figure 3.

As mentioned previously, the same drive shaft 93 operates the twoantenna beam scanning mechanisms represented by the dotted lines 99, 79,and assumed to be of the same construction as the above mentionedAllebach application and built into the antenna assemblies. In theshowing of Figure 1, the eccentric cams 83, 81 on shaft 93 operate thesame scanning mechanism. Since each of the cams 83, 81 has one lobe,while its associated shutter 75A or 75 has two lobes, one opening in theshutter will find the antenna scanning in one direction, the other inthe opposite direction. The azimuth and elevation unblanking switches75A and 67 are shown schematically in Figure l as being cam actuated,being operated by the two-lobed cam 89, for purposes of establishing theunblanking or intensifying voltages represented in Figure 4.

The azel relay switch 69 is operated by the cam 87 on shaft 93 tocontrol current to the circuit switching relays, the function of whichis described hereinafter.

Radar echo signals, when received at the elevation antenna 103 or theazimuth antenna 55, as the case may be, are fed back into the switch 101and passed through the T-R switch 97 into the receiver 57. Receiver 57serves to detect the video and after the video is amplified in the videoamplifier stage 107, it is applied as so-called normal video to thecorrespondingly designated leads 22 in both Figures 2 and 7. Such video,i.e., radar video, derived from echo signals may be applied directly tothe cathode of the cathode ray tube 11 shown in Figure l for purposes ofproducing visual indications; or, such normal video may first bestandardized by applying the same to the video shaper indicated as suchin the block diagram shown in Figure 7 and described in greater detailwith respect to Figure 8. It is understood that other means may be usedfor applying the video to an intensity control electrode of a cathoderay tube.

The cathode ray tube 11 in Figure 1 has a pair of magnetic deflectioncoils 22B, 22A, so arranged as to deflect the associated electronic beamsubstantially parallel to two mutually perpendicular axes, i.e., thesocalled time base" axis which is generally, although not exactlyhorizontal as viewed by the operator, and the so-called expansion axiswhich is generally vertical. In general, each basic trigger pulsedeveloped in synchronizer 31 (Fig. 2) is made to initiate a current waveof sawtooth form through the time base deflection coil 22B and a currentwave of similar form through the associated expansion deflection coil22A, the current in each coil expanding approximately linearly with timeand then returning rapidly to zero. Instead of a linear variation, thisvariation may be logarithmic in character as described in the abovementioned Homer G. Tasker application, Serial No. 175,168, filed July21, 1950, now Patent No. 2,737,654, granted March 6, 1956, and assignedto the same assignee as the present application.

The resulting rate of such sawtoothed current is of course the same as,or a fractional multiple of, the pulse repetition rate of thetransmitted radar pulses and occurs during the expectant period ofresulting echo signals. It will be understood that electrostaticdeflection of the cathode ray beam may be used instead ofelectromagnetic deflection, appropriate modification being made in otherparts of the equipment.

Such sawtooth currents applied to the deflection coils 22B, 22A,however, are modulated at a much lower rate by currents of much lowerperiodicity which are produced by the aforementioned beam angle voltagesshown in Figure 2. Those portions of the voltage indicated in heavylines in Figure 2 only are used to modulate the voltages on a timesharing basis.

These voltages as represented by the curves 61, 63 may vary from plus 2volts at one extreme of the scanning range to plus 52 volts at the otherend. These particular antenna beam angle voltages as mentionedpreviously are used in eflect to modulate an amplitude of the sawtoothvoltage waves developed at the sweep amplifier shown in Figure 2 andapplied at a much higher repetition rate to the expansion coil 22A, forpurpose of oblaining so-called uni-directional or uni-dimensionalmagnitudes in the cathode ray display, in accordance with the principlesset forth in the copending application of Homer G. Tasker, Serial No.680,604, filed July 1, 1946, now abandoned, and assigned to the sameassignee as the present application. On the other hand, the amplitude ofthe sawtooth voltage waves developed at the sweep amplifier and appliedto the other quadraturely acting time base coil 22B is likewisemodulated to a much smaller degree and in a different manner, forpurposes of orientation. Thus the amplitude of the currents applied tocoil 22A is automatically varied in accordance with antenna beam anglevoltage, so that the angle which any particular cathode ray beam makes,corresponds, on an expanded scale, to the antenna beam voltage.

The tube 11 is rendered fully operative for producing visibleindications only when a suitable intensifying voltage is applied to itsgrid 1126, bringing the tube approximately to cut off condition. Arelatively small additional video signal applied to the cathode 112Cthen strengthens the cathode beam, making it momentarily visible on thescreen as a dot, the position of which is determined by the currentsflowing at that particular moment in the set of deflection coils 22A,22B.

For purposes of developing the aforementioned suitable deflectingcurrents in the cathode ray deflection coils 22A, 22B, the sweepgenerating circuit shown in Figure 1 is applied with basic triggersoriginating in the synchronizer 31 and applied to lead 10. Such triggeris applied to the delay multivibrator and blocking oscillator stage A,the output of which is fed to the sweep generating multivibrator stage111A. A negative gating voltage is generated in the stage 111A and fedto the expansion and time base modulator stages 112A, 123A,respectively, and from them in modulated form through the expansion andtime base amplifiers 124A, 125A. The

output of the amplifiers 124A, 125A in the form of essentiallytrapezoidal waves of appropriate amplitude are applied to the expansiondeflection coil 22A and the time base deflection coil 2213,respectively, causing current pulses of substantially linear sawtoothform in the coils. Expansion and time base centering circuits 126A,127A, are also connected to the deflection coils. The modulator stages112A, 123A, for purposes of modulation, receive az-el antenna beam anglevoltages via switches in and n, respectively, of relay K1101.

With the relay unactuated (as shown) the elevation beam angle voltageappearing on the potentiometer resistance 134A is applied through switchm to the expansion modulator 122A; and through potentiometer resistance135A and inverter 135B and switch n to the time base modulator 123A.After completion of the elevation scan, relay K1101 is energized throughswitch 69 breaking the elevation beam angle voltage connections justdescribed, and connecting the azimuth beam angle voltage throughpotentiometer 136A and switch m to the expansion modulator 122A, andthrough potentiometer 137A, inverter 131A and switch n to the time basemodulator 123A.

Thus the degree of modulation of sweep current, and hence the degree ofangle expansion of the display shown in Figure 6 may be separatelyregulated for the azimuth display by adjustment of the potentiometer135A, and for the elevation display by adjustment of the potentiometer134A; and the degree of modulation of the time base sweep current, andhence the apparent angle between the range marks and the time base maybe separately regulated for the azimuth display by adjustment ofpotentiometer 137A, and for the elevation display by adjustment of thepotentiometer 136A.

The centering circuits 126A, 127A in Figure 1 are individually capableof two separate adjustments, one effective when relay K1102 is actuated(azimuth display) and one when the relay is unactuated (elevationdisplay) to determine the position of the points O respectively, inFigure 6. Thus the origins of azimuth and elevation displays areseparately adjustable, the centering circuits automatically respondingto one or the other set of adjustments according to the energizingcondition of relay K1102. A schematic diagram showing a centeringcircuit for this purpose is shown in Figure 5.

The deflection coil 22A in Figure 5 is connected between a 700 voltpositive supply and two parallel circuits, one leading to ground throughtube V-1116, which is the final stage of expansion amplifier 126A, andthe other returning through choke coil L1101 and centering tube V-1l17to a LOGO-volt positive supply. The first of these two circuits feeds todeflection coil 22A, the periodically varying sweep producing component,while the second circuit provides a relatively constant but adjustablecentering current component. The cathode resistor of centering tubeV-lll'l is made up of two parallel connected potentiometers 13 and 15,the movable contacts of which are connected respectively to the normallyclosed and normally open contacts of switch m of relay K1102. A switcharm is connected through grid resistor 17 to the tube grid. The gridbias, and hence the centering current through the tube and through thecoil 22A, thus depends upon the position of relay switch in and isdetermined by the setting of the potentiometer 15 when relay K1102 isactuated (azimuth display) and by the potentiometer 13 when the relay isnot actuated (elevation display). The two displays are thereforeseparately adjustable on the indicator tube by means of the twopotentiometers.

The time base deflection coil 22B is provided with centering circuitwhich is identical to that in Figure 5 and functions in a like manner,controlled by switch n of relay K1102. In fact, by appropriate changesof the numerals and lettering Figure 5 may be considered to illustratethe time base centering circuit. The potentiorrteters then provideseparately adjusted ordinary elevas tion and azimuth displays withrespect to the horizontal positions.

It is noted that the preferred interrelationship of the two displays inFigure 6 is such that the series of corresponding range marks of the twopatterns lie in a straight line so that the two aircraft images 38A, 39Aalways lie in a line just parallel to the range mark lines, one directlyabove the other.

The azimuth and elevation displays shown in Figure 6 are limited so thatthey appear as shown, such pattern clipping or limiting being producedby operation of the pattern clipper or limiter 40A shown in Figure 1.Such sweep limiter 40A forms, per se, no part of the present inventionand may be the one described and claimed in the copending patentapplication of Raymond B. Tasker, Serial No. 212,163, filed February 21,1951, and assigned to the same assignee. In general, the output of sweeplimiter stage 40A is a negative-going gating voltage 40B applied to thefirst anode 19 of the cathode ray tube 11. Such negative-going gatingvoltage 40B is used for darkening, i.e., blanking out, the indicationswhich may be otherwise visible. Such blanking occurs during undesiredperiods of sweep as now described specifically.

The azimuth display, which is preferably the lower one, is blanked orclipped or limited, above a horizontal line LM which extends parallel tothe runway axis A and at a sulficient distance above it to allow forexpected errors in the azimuth angle of approaching aircraft. In theelevation (upper) display, a section is cut out or clipped, such sectionbeing below the horizontal runway axis 0 6 and to the right of a shortgenerally vertical line K]. This line K] is located just to the left ofand parallel to the upper limiting sweep path 0 L of the lower azimuthdisplay. The region thus eliminated from the elevation displaycorresponds to space below the runway level.

Besides serving to produce this desired clipping in the visual display,the negative-going gating voltage 408 developed in the limiter stage 40Ais useful in the automatic tracking system shown in block form in Figure7 for limiting the time during which video is available in suchautomatic system. For that purpose gating voltage 408 is applied asshown therein to the Video Shaper" for purposes of limiting the timeduring which standardized video is produced in the manner describedhereinafter.

As shown in Figures 1 and 7 the input to the sweep limiter 40A is: (l) atrigger derived from the basic trigger appearing on lead 10; (2) theazimuth and elevation angle coupling voltages on leads 18 and 20respectively; and (3) the az-el relay voltage on lead 16. It isunderstood that this negative gating voltage 403 appears at variabletimes along the time axis depending upon the magnitude of either theazimuth or elevation beam angle voltage, whichever one at thatparticular time is efiective.

The purposes of the switches 300A, 300B shown in Figure 1 are fullydescribed in the above mentioned application of Homer G. Tasker and forthe present instance may be considered to remain closed.

It is observed further in connection with Figure 1 that the sweepmultivibrator 111A generates a positive-going gating voltage 21 of aduration substantially equal to the time duration of the cathode beamsweep and such positrve-going gating voltage is applied to the secondmixer stage 23 to produce the wave form 25. This wave 25 comprisespulses of sweep frequency added to the longer azimuth and elevationgates which are developed in the first mixer stage 27 and shown also inFigure 4. This composite wave 25 is applied to the cathode ray grid1126, bringing the tube up to the point of cut ofi during each sweep. Bythis expedient the cathode ray tube is conditioned for producing visualindication only during those times when video signals are beingexpected.

The range marks 40, 41, 43, 45, 47, and 49, shown in Figure 6, aredeveloped by the range mark generator 41A (Fig. 1) in accordance withbasic triggers applied to such stage from lead 10. The range marksdeveloped in stage 41A are applied to the cathode 112C.

It is observed that the display shown in Figure 6 includes sectorsdefined by the so-called V-follower lines 50A, 51A, and 52A, 53A, whichsectors are developed using the apparatus connected to the leads inFigure l marked Az Servo Data No. l, Az Servo Data No. 2, and El ServoData No. 1 and El Servo Data No. 2.

Also Figure 6 shows the glidepath course line 149A and runway courseline 150A. These two course lines may be developed electronically byapparatus described and claimed in copending application of Raymond B.Tasker and Burton Cutler, Serial No. 222,512, filed April 23, 1951, nowPatent No. 2,832,953, granted April 29, 1958, and assigned to the sameassignee; or preferably these lines are obtained using the cursorgenerator illustrated in Figures 24 and 25 herein.

Purpose and function of apparatus The apparatus described hereincombines the functions of (1) aircraft acquisition, (2) automatictracking, and (3) error computation and control signal transmission.

The controlled aircraft is equipped with suitable radio equipment and anautopilot with automatic approach coupler. This equipment may be used asan automatic ground controlled approach system (AGCA) for thesimultaneous guidance of two or more aircraft during their approach to agiven runway adjacent to which radar equipment is located for scanningthe approach zone.

The radar system incorporates two antennas, one for scanning theapproach zone in a vertical plane, and the other antenna scanning thesame approach zone in a horizontal plane. Vertical scan is from 1 to +6while horizontal scan is in the order of 20''. In a system of thischaracter, an approaching aircraft is first located by conventionalsearch radar, using, for example, a plan position indicator (P.P.I.) andis then directed by radio communication to the correct position forentry into a predetermined ideal glidepath (vertical plane) and courseline (horizontal plane). The final approach along such ideal glidepathand course line is indicated upon the face of a cathode ray tube and theactual course of the aircraft is visually compared with that of an idealapproach, such ideal approach, i.e., ideal glidepath and ideal courseline, being developed electronically by a so-called cursor generator.

In prior art systems of this character, radio communication is used todirect the aircraft along such ideal glidepaths and course lines; but inaccordance with the present invention, means are provided for developingand transmitting to the aircraft control signals which arerepresentative of the deviation of the aircraft from such glidepath andcourse line for purposes of maintaining, or tending to maintain, theflight of such aircraft along such glidepath and course line.

For accomplishing such automatic control of aircraft, the AGCA systemdescribed herein is such as to receive information from conventional GCAradar equipment relative to the range azimuth and elevation positions ofthe approaching aircraft and to compare these positions with an idealpredetermined glidepath. The result of this comparison, in the form oferror signals, is electronically computed and automatically sent to thecontrolled aircraft via very high frequency radio communication. AGCAairborne equipment receives this information (correction signals) andinterprets it in the form of control voltages, which are applied to theaircrafts autopilot approach coupler.

The range of this automatically controlled approach is fromapproximately 8 miles from the given landing field to a point of releasefrom the system, known as touchdown. This point of release, ortouchdown, is at an altitude of approximately 50 feet above the givenlanding strip; and at such a position of altitude that the pilot mayassume control for the actual landing operation. during the last fewseconds of the landing.

Prior to the establishment of flight control of an approaching aircraft,communication between the AGCA installation and pilot of the incomingplane may be elfected via a conventional transmitter receiving system inthe VHF band in the region of megacycles.

Briefly, in operation of the AGCA system, the search: radar operator,using the display of the conventional search radar (P.P.I.), tracks theaircraft to a proper posi tion altitude of the AGCA final approach. Theentry into the AGCA system is along an on course approach lineat adistance of approximately 10 miles and at an ele vation of approximately2800 feet above the airfield.

In the meantime, the radar equipment, being en-- ergized, is in itssearch" function or condition in which a slow search sweep voltage isperiodically developed for searching a radar echo from the approachingaircraft. As a matter of fact, coincidence of a radar echo from suchaircraft with such slow search sweep voltage notifies the system of anapproaching aircraft; and thereupon the tracking unit, illustrated inFigure 13, automatically switches from such search function or conditionto a track condition and displays the range and speed of the incomingaircraft. Simultaneously, upon switching from such search to trackfunction, the AGCA transmitter is turned on" and a subcarrier on atransmitted wave, containing a so-called channel select key, istransmitted to the approaching aircraft. At a given range, or upondirections from the ground via conventional radio transmission, thepilot of the approaching aircraft renders effective his airborne decoder(signal data converter) by actuating a switch.

Actuation of such switch starts the search drive motor of the airbornedecoder, and the output of the AGCA airborne receiver is searched for anAGCA subcarrier. At intervals of 25 seconds, the AGCA ground transmitteris automatically interrupted for a one-second period. This interruptionconstitutes interrogation.

If upon the interrogation" the airborne decoder has located thetransmitted subcarrier, the signal interruption causes the detector tosend a 4500 cycle per second confirmation signal to the ground via theairborne transmitter. This confirmation signal is received by the AGCAreceiver and serves to energize relay windings to apply a +28 voltso-called "contro signal to a common bus of the ground equipment.

At the time the range tracking unit in Figure 13 automatically switchesfrom its search function to its track" function as described above, aso-called tracking on" signal developed in the range tracking unit isapplied to the computer unit illustrated in Figures 18A and 18B, so thata computer unit is conditioned to compute the error, if any, of theaircraft from the ideal glidepath and ideal course line.

Upon development of the confirmation control signal resulting fromconfirmation, the AGCA transmitter is turned on to transmit to theaircraft the error signals computed by the unit shown in Figures 18A and18B, as well as certain other information. Such error signals, i.e.,azimuth and elevation control signals, as well as a signalrepresentative of the instantaneous range of the aircraft, is used tomodulate the subcarrier transmitted to the aircraft, to provide theautopilot with correction signals for on course approach and providingthe pilot with visual display instantaneous range from touchdowninformation.

The data, including control signals for elfecting flight of theaircraft, as well as other control signals, are transmitted from theground to the aircraft by the use of a subcarrier on the transmittedwave.

The AGCA system as developed includes a frequency spectrum whichencompasses a carrier width sulficient for the control of six aircraftsimultaneously. For this pur-

